ABSTRACT Osteoid osteoma is a painful localized bone tumor that occurs most commonly in children and adolescents between 5-25 years of age. It accounts for 14% of all benign bone tumors. Symptoms can include severe bone pain and deformity, limb length discrepancy, and scoliosis. The current standard of care definitive treatment is with CT-guided radiofrequency ablation (CT-RFA), which has replaced open surgery. During CT-RFA, a bone drill is placed through the skin, muscle and bone into the osteoid osteoma using a CT imaging guidance. An RFA needle probe is then used to burn the osteoid osteoma. Although CT-RFA is less invasive than open surgical resection, potential complications include bleeding, infection, skin and muscle burn, and nerve injury from drilling through tissue and heating along the RFA probe. Furthermore, the use of CT imaging required for guidance exposes patients and operators to ionizing radiation, which can have potential long-term negative effects, especially for growing children. An optimal image-guided therapy for treatment of painful localized tumors such as osteoid osteoma would ideally be completely non-invasive, precise, and radiation-free. It would be performed without any skin incisions or bone drills, limiting potential collateral tissue damage and complications. It would be performed with real-time imaging feedback that could be used to prevent thermal injury to nearby critical structures and also to confirm treatment success. Magnetic resonance imaging-guided high intensity focused ultrasound (MR-HIFU) is a novel image-guided therapy which offers such treatment. Our group has performed the first Phase I clinical trial in the US to evaluate MR-HIFU treatment for painful osteoid osteoma in a pediatric cohort (PI Sharma, NCT02349971). The results show that MR-HIFU is feasible, well tolerated, and can be safely performed in children with painful osteoid osteoma. Based on our encouraging preliminary results, we hypothesize that this completely non-invasive and radiation-free MR-HIFU therapy will be clinically effective for treatment of painful osteoid osteoma; with clinical success comparable to CT-RFA. To test this hypothesis, we propose an early Phase II clinical trial designed to perform a thorough evaluation of the safety and efficacy of MR-HIFU ablation in patients with painful osteoid osteoma. We will also closely evaluate MR thermometry, intrinsic contrast imaging, and diffusion-weighted MR imaging obtained during the course of treatment and determine if these imaging changes can depict ablation zone without the use of intravenous injections of gadolinium-based contrast agents and serve as imaging biomarkers of treatment efficacy.